Balancing for gravitational effects is usually required for hinged and/or articulated arm robots when such robots are likely to be activated manually or by some lower level power source. Such an occurrence typically takes place during teaching. Elimination or reduction of the effects of gravity allow the use of smaller power sources which reduces energy utilization and allows for better stability of servo-controlled mechanisms, such as the robot arm. With a balancer mechanism articulated arm robots can be designed so that they can be manually led through their desired tasks without the use of a prime mover and the complexity associated with the controller of the robot. As a result the robot arm can be manually led through each desired task under low level power requirements.
The prior art shows numerous gravity balancing mechanisms used on articulated arms and hinge mechanisms. One such arrangement utilizes counterweights for balancing the robot arm. However, the use of counterweights is oftentimes objectionable because of the added mass and resulting increase in arm inertia. For example, the inertia of a counterweight must be overcome every time the robot arm is to be moved in a different direction. Braking and change of direction of the robot arm is subject to inertial deceleration and acceleration forces due to the counterweights.
The following prior art patents disclose the use of counterweights as counterbalance mechanisms: the U.S. patent to Roselund No. 2,344,108; the U.S. patent to Cooper No. 3,543,989; and the U.S. patent to Le Rouzo No. 4,402,646.
Other prior art patents disclose the use of hydraulic and pneumatic balancers of both the active and passive type. Active balancers require an external power source to supply or absorb the balancing energy. Passive balancers store and release the balancing energy as required. Many of such hydraulic or pneumatic counterbalance mechanism are relatively complex and costly. For example, the U.S. patent to Panissidi, U.S. Pat. No. 4,229,136 discloses an air pressure counterbalance system including an air-driven piston operated in the direction of the gravity axis as the manipulator hand is raised and lowered. The weights of different tools are programmed into computer memory and thereafter an air pressure regulator adjusts the counterbalancing force depending upon which tools are used by the manipulator.
Other U.S. patents which disclose hydraulic or pneumatic counterbalancing mechanisms include the U.S. patents to Sack et al. No. 3,370,452 and Davini No. 4,300,198.
When balancing is required within a small angle or within a single quadrant (i.e. from a horizontal to vertically upward orientation) a level of balancing can be obtained with a spring or a passive pneumatic balancer. The following prior art patents disclose spring balancers which are useful within small angles of movement: the U.S. patent to Flatau No. 3,391,804; the U.S. patent to Stolpe No. 4,024,961; the U.S. patent to Belyanin et al No. 4,259,876; the U.S. patent to Vertut No. 4,283,165; and the U.S. patent to Susnjara No. 4,378,959.
One objection to the use of conventional spring balancers is that the spring normally can only apply a variable force and there is normally no continuous and/or adequate compensation for the effect of varying gravitational forces. Also, it is inherent in most spring balancing methods that complete balance is possible only for one or two configurations of the arm and spring combination. As the robot arm moves away from that configuration in either of two possible directions, an unbalance is generated and progressively changes until the arm approaches a neutral orientation of zero gravitational moment.
Spring balancers currently do not provide adequate balancing over extended angular movement of the robot arm. Because of this, oftentimes there are high actuation power requirements to overcome the effects of gravity on the robot arm, especially if the robot arm is moved upwards. Such high actuation power requirements present a safety hazard if the mechanism should fall under the force of gravity when motor power is shut off. Consequently, such mechanisms are usually provided with brakes to alleviate that potential danger.
Spring stiffness, initial tensioning and anchor point location can be adjusted to give a higher degree of balance within a small angular displacement of the arm and also limit the maximum value of the unbalanced moment and/or its direction. Beyond that displacement the degree of unbalance grows relatively rapidly.
Despite the relative simplicity and relative inexpensiveness of conventional spring and passive pneumatic balancers, the balancers have generally not been able to overcome their current angular limitations.